fix: pass USE_CELERY_FALLBACK to containers and show real error in failure UI

- docker-compose.yml: add USE_CELERY_FALLBACK env var to api and worker
  services so cloud_run_dispatch uses Celery on optical-dev
- JobDetail.tsx: show actual error message instead of generic
  "Processing failed at ." when failure step is unknown; also show
  job.error string when no structured failure object exists

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -139,6 +139,9 @@ services:
       COST_TRACKER_API_KEY: ${COST_TRACKER_API_KEY:-}
       COST_TRACKER_SOURCE_APP: ${COST_TRACKER_SOURCE_APP:-video-accessibility}
       COST_TRACKER_ENABLED: ${COST_TRACKER_ENABLED:-true}
+
+      # Pipeline dispatch mode
+      USE_CELERY_FALLBACK: ${USE_CELERY_FALLBACK:-false}
     volumes:
       - ./secrets:/secrets:ro
       - api-logs:/app/logs
@@ -179,6 +182,7 @@ services:
       # When set, CPU-intensive operations are offloaded to Cloud Run
       WHISPER_SERVICE_URL: ${WHISPER_SERVICE_URL:-}
       FFMPEG_SERVICE_URL: ${FFMPEG_SERVICE_URL:-}
+      USE_CELERY_FALLBACK: ${USE_CELERY_FALLBACK:-false}
 
       # Auth (required by Settings class even though worker doesn't use it)
       JWT_SECRET: ${JWT_SECRET}

frontend/src/routes/jobs/JobDetail.tsx

@@ -87,10 +87,13 @@ const ProgressIndicator = ({ status, failure }: { status: string; failure?: JobF
             </div>
           ))}
         </div>
-        <div className="mt-3 pt-3 border-t border-gray-200">
-          <p className="text-xs text-red-600">
-            Processing failed at {failedStepLabel}. Check error details for more information.
+        <div className="mt-3 pt-3 border-t border-gray-200 space-y-1">
+          <p className="text-xs text-red-600 font-medium">
+            {failedStepLabel ? `Failed at: ${failedStepLabel}` : 'Processing failed'}
           </p>
+          {failure?.message && (
+            <p className="text-xs text-red-500 break-words">{failure.message}</p>
+          )}
         </div>
       </div>
     );
@@ -706,7 +709,9 @@ export function JobDetail() {
                     ? 'TTS generation failed. Fix audio description text before retrying.'
                     : job.status === 'render_failed'
                     ? 'Video rendering failed.'
-                    : 'Processing failed.'}
+                    : job.error
+                    ? job.error
+                    : 'Processing failed. Retry or escalate to support.'}
                 </p>
               )}
               <div className="flex gap-2">